1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image using an electrophotographic system.
2. Description of the Related Art
An image forming apparatus according to the related art forms a color image using toner of four colors of yellow, magenta, cyan, and black. As a developer, a two-component developer is used which contains nonmagnetic toner particles (toner) and magnetic carrier particles (carrier). Japanese Patent Laid-Open No. 2004-177928 and Japanese Patent Laid-Open No. 9-34243 disclose a color image forming apparatus in which the colors of an output material are stabilized by stabilizing the density of each color.
An image density gradually changes as the number of printed sheets increases. However, the level of the change is influenced by the contents of the formed image as well as by the number of printed sheets of the formed image. For example, an image with a relatively high density in which a lot of solid portions occur or which contains a large number of characters, and an image with a relatively low density which contains fine lines or a small number of characters are greatly different in toner consumption even when the number of formed images is the same.
In Japanese Patent Laid-Open No. 2004-177928, a small image (patch image) for a test is formed on the image bearing member at a timing based on toner consumption information and then density control factors influencing the density of the image are optimized based on the density of the patch image.
Further, in a case where images consuming little amount of toner are continuously output, the replacement of the toner is performed occasionally, so that the developer is repeatedly rubbed and stirred for a long term. The toner contained in the developer which has been repeatedly rubbed and stirred for a long term may have an irregular shape, or the distribution of the particle diameter is biased. In addition, an external additive such as titanium oxide particles which is added for the purpose of improving fluidity of the developer is implanted in the surface of toner. As a result, there may be degradation such as a decrease in the fluidity of the developer, and thus it is difficult to obtain an image having a desired image quality.
In addition, even though the electric charges of the toner are stabilized through several times of slidable rubbing, as several times of slidable rubbing are repeated, the electric charges are gradually increased, so that the electric charge amount becomes greater than a predetermined value. In this way, if the electric charges of the toner are increased, the amount of toner adhered on a latent image having the same potential difference with respect to a developing sleeve which is formed on the photosensitive drum is reduced compared with an initial state. When an image is output, the density is deteriorated and granularity of the low density portion is worsened, so that the image quality is degraded.
In addition, in a case where the density of the output image is reduced due to degradation in the developer with time, the densities of colors are separately adjusted while securing a maximum density by changing conditions such as a charging potential or a developing bias, power of laser beams, or time for emitting laser beams. In order to secure the maximum density, it is necessary to increase a difference potential (hereinafter, referred to as a contrast potential) between a potential VL of an image area and an average potential of potentials Vdc applied to the developing sleeve.
Further, in Japanese Patent Laid-Open No. 9-34243, a decrease in the density which occurs when the toner consumption is reduced is suppressed by forcibly replacing the toner in a development device.
However, in Japanese Patent Laid-Open No. 2004-177928 and Japanese Patent Laid-Open No. 9-34243, the density of each color is stabilized with an individual countermeasure about the development device for each color of yellow, magenta, cyan, or black, thereby making the color of the output material stabilized. For example, in a case where the density is increased only in a development device of a given color, when the density is corrected such that the increased density of the development device becomes equal to those of the other normal development devices, it is necessary to lower the contrast potential of the development device which has been changed in the density. However, if the contrast potential of only the development device which has been changed in the density is lowered to depart from a predetermined range, the development γ (the development performance with respect to the development potential) of the development device becomes too strong. In other words, a large amount of the toner is consumed at a small contrast potential. For this reason, in a case where the potential of the photosensitive drum is changed in the surface thereof, the variation ratios of the densities of high-order colors such as a second-order color of red, blue, or green are different in each color, so that a color tone of an image having high-order colors varies.
In addition, in a case where the density is reduced only in a development device of a given color, when the density is corrected such that the reduced density of the development device becomes equal to those of the other normal development devices, it is necessary to increase the contrast potential. However, in a case where the contrast potential is greatly increased, an adverse effect such as carrier attachment occurs on an image.
In this manner, in a case where the density variation occurs in one development device, if a process is performed to correct the density of the development device in which the density variation occurs, the density correction process may lead to a problem.